1. Field of the Invention
The present invention relates to a method of forming a fine copper conductor pattern in an LSI (large scale integration) circuit, and more particularly to a method of forming a finer copper conductor pattern by a dry etching process which employs a gas containing chlorine.
2. Description of the Prior Art
It has heretofore been customary to employ circuit conductors or interconnections of aluminum in LSI circuits. As LSI circuit conductors or interconnections become finer and finer, however, the aluminum is less resistant to stress migration and electromigration, posing a problem as to the reliability of semiconductor devices. Another matter of concern is that as semiconductor devices operate at higher speeds, the processing speed of a semiconductor device is governed by the delay caused by circuit conductors or interconnections. It is therefore necessary to use circuit conductors or interconnections of a material having a lower electric resistance in order to realize higher-speed semiconductor devices.
To meet the above demands, an attempt has been made to use copper as a conductor or interconnection material for further semiconductor devices. However, it is difficult to form fine conductor patterns of copper for LSI circuits using the presently available dry etching process for forming fine conductor patterns of aluminum, and hence the present dry etching process entails drawbacks with respect to the formation of fine copper interconnections. The reasons why the present dry etching process cannot be used to produce fine copper conductors are as follows: In the normal dry etching process, a halogen decomposed by a plasma is adsorbed to the surface of a substrate to be etched, forming molecules that can easily be desorbed from the substrate surface. When subjected to ion impact, these molecules are desorbed from the substrate surface, thus etching the substrate. At this time, since the ions are applied perpendicularly to the substrate, the etching process is anisotropic in that side walls on the substrate are not etched. When a copper substrate is etched by the normal drying etching process, however, no etching progresses because a copper halide is not desorbed from the surface of the copper substrate. This is because the equilibrium vapor pressure of the copper halide is much lower than that of aluminum, and almost no ion-assisted effect occurs. As a result, when the substrate is exposed to a Cl.sub.2 plasma at about room temperature, only a layer of CuCl.sub.2 is formed on the copper surface, and no etching occurs. One solution has been to heat the copper substrate to 250.degree. C. or higher to accelerate desorption of the copper halide to etch the copper surface.
The above conventional process of forming fine copper conductors is not compatible with the dry etching process because the photoresist which is presently used mainly to form aluminum conductors or interconnections cannot be used as a mask for the reason that the photoresist would be hardened due to the high temperature of the substrate in etching the copper surface. Since the substrate temperature is high, chlorine is easily thermally diffused into copper conductors which are being etched, with the result that many chlorine molecules are present on the surface of the film and within the film after the etching process is finished. These chlorine molecules on the surface of the film and within the film react with moisture in the air, corroding the copper conductors and lowering the reliability of the semiconductor device.
When the present dry etching process is employed to form fine copper conductors, the etching reaction progresses primarily thermally, rather than being ion-assisted. Specifically, molecules produced on the copper surface during etching are desorbed by procuring thermal energy from the substrate. In the case where there is an ion-assisted effect available for desorption of adsorbed molecules by collision with ions, etching progresses in regions where the ions are applied, and no etching reaction takes place in other regions. Accordingly, it is possible to form fine conductor patterns with an anisotropic property which is produced by the linearity of applied ions. In the dry etching of copper where molecules are desorbed principally by the thermal energy from the substrate, etching progresses also on side walls. Consequently, an isotropically etched configuration is produced, and it is difficult to obtain an anisotropic property that is required to form fine conductor or interconnection patterns by dry etching of copper.